1. Field of Invention
The present invention relates to systems and methods for locating gas hydrate deposits.
2. Discussion of Related Art
Gas hydrates are a class of clathrate (lattice-like) compounds in which individual small molecules, commonly in the gas phase at room temperature and pressure, occupy sites within a solid crystalline matrix of water molecules. In natural gas hydrate reservoirs, the guest molecules are either pure methane or a mixture of compounds comprising natural gas. For gas hydrate deposits to form, a source of gas is required. Seeps of natural gas, generally comprising methane, are common in many parts of the world. Natural gas hydrate deposits are found in both terrestrial and marine environments. Terrestrial hydrates accumulate in and under permafrost in arctic regions. Marine gas hydrates may be found trapped in subseafloor sediments in water depths of at least about 500 meters (m).
Gas hydrates form at elevated pressure and reduced temperature. The gas hydrate stability zone in subsea sediments can be delineated on a temperature versus depth (pressure) profile with respect to the hydrothermal gradient (for subsea gas hydrates), geothermal gradient and clathrate phase boundary, as shown in FIG. 1. Referring to FIG. 1, there is illustrated a phase diagram showing the pressure-temperature dependence of methane-hydrate stability in a subsea environment. On the vertical axis, pressure is represented in terms of depth (in meters) below the sea surface (this conversion assumes the normal ocean and pore pressure gradient of 10 MPa/km). On the horizontal axis is temperature in degrees Celsius. The seafloor is indicated as dotted line 100. The geothermal gradient is shown as line 104 and the hydrothermal gradient is shown as line 106. Hydrate can exist when the temperature at a given pressure is less than the hydrate transition temperature at that pressure. Line 102 illustrates the hydrate-gas phase boundary as a function of temperature and pressure. For temperature and pressure conditions below this line, methane may exist in the hydrate form. For temperature and pressure conditions above this line, methane may exist in the gas phase. The position of the hydrate phase boundary is primarily a function of gas composition, but may also be controlled by pore fluid composition (e.g. presence of salts), pore size, and possibly sediment mineralogy. For example, adding sodium chloride to the water may shift line 102 to the left, while adding carbon dioxide, hydrogen sulfide and other hydrocarbons may shift line 102 to the right.
Hydrates are stable above the isotherm at which the geothermal gradient 104 of the solid earth crosses the phase line 102, typically several hundred meters below the seafloor. This is the base of the gas hydrate stability zone 108. The upper boundary of the gas hydrate stability zone 108 may be by the intersection of the hydrothermal gradient 106 and the hydrate phase boundary 102. Hydrothermal and geothermal gradients are locality dependent, and can differ markedly with geographical location and tectonic setting. Since natural gas hydrates are less dense than water, they are not found in the water region of the gas hydrate stability zone. This is because any hydrate forming in the water floats to the surface and decomposes. However, they are effectively trapped in subseafloor sediments.
Hydrates are also stable in a band of depths below the land surfaces in arctic regions, overlapping and below the range of permafrost stability. FIG. 2 illustrates a gas-hydrate phase diagram defining the gas hydrate stability zone (GHSZ) in a terrestrial arctic environment. Gas hydrate exists when the temperature is less than the gas hydrate transition temperature (i.e., the temperature at which the phase boundary between the gas and hydrate forms is crossed) at the local pressure. The gas-hydrate phase boundary is illustrated as line 110, dotted line 112 illustrates the geothermal gradient, and line 114 illustrates the fresh water-ice phase boundary. On the vertical axis, pressure has been converted to depth below ground level assuming the normal pore pressure gradient of 100 bar/km (10 MPa/km). Terrestrial gas hydrate exploration programs have been successful in several areas, such as Siberia, the Canadian arctic, and the North slope of Alaska.
Over one hundred occurrences of gas hydrates on continental margins and in inland seas have been documented, suggesting that gas hydrates are widespread in deep water marine environments. In most cases, the location and areal extent of hydrate deposits are estimated from a peculiar seismic signature of gas hydrate presence called the bottom simulating reflector (BSR). The BSR is seen in many marine seismic images, running parallel to, and several hundred meters below, the seafloor, and approximately coincides with the base of the gas hydrate stability zone. Surveys of bottom simulating reflectors found in various parts of the world suggest that the amount of organic carbon stored in undersea gas hydrates is very large. A widely quoted estimate predicts that there may be twice as much organic carbon in gas hydrates as there is in all recoverable and unrecoverable conventional fossil fuel sources, including natural gas, coal and oil. In addition, marine gas hydrates are thought to be primarily found on continental slopes, which are usually within the exclusive economic zones of coastal nations and near consumers in the United States, Japan, India and elsewhere.
However, the actual amount of gas hydrate stored in marine sediments is highly uncertain. Although there have been several major drilling campaigns (e.g., in regions offshore of South Carolina and Oregon), and a few significant concentrations have been found in limited depth intervals, gas hydrate is generally dilute throughout the gas hydrate stability zone in most locations that have been drilled.
Another characteristic of the seismic response to gas hydrate is amplitude blanking within the gas hydrate stability zone. “Blanking” refers to a depth interval with low amplitude reflections in a seismic image, as shown, for example, in FIG. 3. Referring to FIG. 3, a region 116 having low amplitude reflections can be seen between the seafloor 100 and the bottom simulating reflector 118. Appearances of region 116 in seismic images is referred to as amplitude blanking. A variety of explanations have been proposed to explain blanking. One explanation that has attained widespread support holds that hydrates, which increase the acoustic velocity of unconsolidated sediments, are most likely to form in high porosity (i.e., low velocity) strata, thus reducing the acoustic contrast with neighboring strata. Blanking has also been explained by the disruption of sedimentary stratigraphy in marine environments thought to harbor hydrate deposits. Another explanation suggests that destructive interference from vertically displaced reflectors within the Fresnel zone reduces the amplitude of the seismic reflections. A fourth explanation attributes blanking to the presence of liquid and gas migrating upwards through conduits which may be connected to deeper faults. Although any of these explanations is plausible, establishing a connection between any of them and an exploration strategy has proved difficult.
A number of theoretical studies have described the principles of seafloor electromagnetic surveys. However, for the most part, the emphasis of such work has been on one-dimensional earth models in which conductivity changes only with depth. Electromagnetic field studies have been carried out offshore of Vancouver and offshore of Oregon, where seismic and drilling programs has previously indicated that gas hydrate was present. However, data processing for all hydrate surveys has assumed horizontally stratified earth, in which the electrical conductivity is isotropic within each horizontal layer.